This invention relates to a failsafe system in an automatic transmission.
Vehicle automatic transmissions having a gear shift mechanism, a plurality of frictionally engaging devices and a hydraulic control device to selectively engage the frictionally engaging devices to set various gear stages are well known in the art.
The conventional frictionally engaging device is comprised of two sets of relatively rotatably friction plate elements and a hydraulic servo device for driving the friction plate elements. When oil pressure is fed to the hydraulic servo device, the two sets of the friction plate elements are strongly urged together, so that the friction plate elements frictionally engage with sufficient force to transmit torque therebetween.
The lower limit for the control oil pressure to the frictionally engaging device of the type described is the pressure wherein:
(1) all of the gear stages in all of the shift positions can be achieved irrespective of engine load (an engine output), vehicle speed or the like, i.e. the pressure giving rise to a torque capacity which precludes slippage of the frictionally engaging devices during non-shift running: (2) the pressure giving rise to a torque capacity resulting in completion of shifting within a predetermined period of time so as not to damage the frictionally engaging devices during shifting from excess slippage.
In general, the working oil pressure to the frictionally engaging devices is a line pressure. In view of the above-described lower oil pressure limits, line pressure has heretofore been controlled in accordance with a value normally regarded as typifying an engine load, such as, for example, engine throttle opening. More specifically, the control is carried out such that line pressure increases with increasing engine loads.
In prior art devices, a throttle pressure related to the throttle opening is introduced into a control port of a primary regulator valve to control the line pressure. This throttle pressure has been generated by a throttle valve, to which a resilient force increasing with accelerator pedal depression is applied. In recent years, electronically-driven automatic transmission have been developed, whereby essential portions of the control circuit implement electronic circuitry. In the electronically-driven automatic transmissions of this type, information on the throttle opening is processed in the form of an electric signal, whereby the line pressure is controlled in response to an electric signal relating to the throttle opening (For example, in Japanese Utility Model Kokai (Laid-Open) No. 125555/1981).
When it becomes possible to control the line pressure or the throttle pressure by a command from a computer as described above, highly detailed control can be carried out as suggested in a related Japanese Patent Application No. 263131/1985 (filing date: Nov. 22, 1985) for example. More specifically, during non-shift running, the line pressure can be lowered as much as possible in accordance with engine load, vehicle speed and the like, with an appropriate extra margin of safety being kept, to thereby avoid wasteful powerloss suffered by a pump (precise control of a condition 1) for determining the lower limit of the aforesaid control oil pressure.
However, when control oil pressure given to the frictionally engaging devices is controlled by a command from a computer in response to input signals from the various sensors as described above, if there occur malfunctions in a sensor system, short-circuits in the sensor system and a computer input system, unexpected leakage of oil pressure in a hydraulic control device of the automatic transmission, unexpected rise in engine output, and the like, then slips occur in the frictinally engaging devices, whereby the gear stage cannot be suitably maintained, thus possibly deteriorating durability of the frictionally engaging devices.